Refactor Head.Logic and document individual event processing steps in prose

[ch1bo]

Splitting the main update into functions and haddock them individually for easy consumption by all readers.

This has been started, but dropped because of being too formal / cryptic over here: #346

In this ticket, we want to revisit this, but document it in prose!

So the goal is, to have the BIG Hydra.HeadLogic.update function broken down into easier digestible chunks of handling individual events which can then be related to the formal spec later.